Protein pathology is a common denominator in the etiology or pathogenesis of many medical disorders, ranging from malfunction of a mutated protein, to pathological gain of function where a specific protein acquires a novel property, which renders it toxic. Conceptually, inhibition of the synthesis of these of proteins by gene therapy may hold promise for patients having such protein anomaly.
One of the major advances of recent years is the concept of silencing a specific gene by RNA interference, using small interfering RNA (siRNA). RNA interference is based on short (≈19-27 base pairs), double-stranded RNA sequences (designated siRNA), capable of acting, in concert with cellular biological systems [among others, the Dicer protein complex which cleaves double-stranded RNA to produce siRNA, and the RNA-induced silencing complex (RISC)], to inhibit translation and mark for degradation specific mRNA sequences, thus inhibiting gene expression at the translational stage. The use of antisense oligonucoleotide (ASO), being a short sequence (usually 13-25 nucleotides) of unmodified or chemically modified DNA molecules, complementary to a specific messenger RNA (mRNA), has also been used to inhibit the expression and block the production of a specific target protein.
However, albeit the tremendous potential benefits of such approaches for medical care, delivery of such macromolecules into cells remains a substantial challenge, due to the relatively large and highly-charged structures of oligonucleotides (for example, siRNA has an average molecular weight of 13 kD, and it carries about 40 negatively-charged phosphate groups). Therefore, trans-membrane delivery of oliogonucleotides requires overcoming a very large energetic barrier.
The membrane dipole potential is an electric potential that exists within any phospholipid membrane, between the water/membrane interface and the membrane center (positive inside). It is assumed to be generated by the highly ordered carbonyl groups of the phospholipid glyceryl esteric bonds, and its amplitude is about 220-280 mV. Since the membrane dipole potential resides in a highly hydrophobic environment of dielectric constant of 2-4, it translates into a very strong electric field of 108-109 V/m. Conceivably, the membrane dipole potential and related intra-membrane electric field are highly important for the function of membrane proteins, determining the conformation and activity of membrane proteins. However, to the best of our knowledge, to date, the dipole potential has not been recruited for drug development.
Various methods have been developed for delivery of macromolecules such as oligonucleotides or proteins across biological membranes. These methods include viral vectors, as well as non-viral delivery systems, such as cationic lipids or liposomes. However to date, use of these methods has been largely limited to applications in vitro, or to focal administration in vivo, for example, by direct injection into the eye or direct administration into the lung. Efficient delivery has also been achieved to the liver. Among these methods, electroporation is an effective and widely-used method for delivery of macromolecules in vitro. According to this method, an external electric field is applied to a cell suspension, leading to collision of charged target molecules with the cell membrane, subsequent temporary and focal membrane destabilization, and consequent passage of the macromolecules into the cell. However, as described above, electroporation is mainly used in vitro. Electroporation in vivo encounters limited success, and was attempted only to specific organs (e.g., muscle, lung), where external electrodes could be inserted into the target organ.
In conclusion, delivery of macromolecules, such as oligonucleotides or proteins through cell membranes, or through other biological barriers, such as the Blood-Brain-Barrier, Blood-Ocular-Barrier or Blood-Fetal-barrier still presents a substantial unmet need, and systemic delivery of therapeutic macromolecules, still remains a huge, unaddressed challenge.